1. What is the Hubble classification theme? Which category includes the biggest galaxies? Into which category do the smallest galaxies fall? Which type of galaxy is the most common?
2. Describe how the spiral density wave theory accounts for the existence of spiral arms in some spiral galaxies.
3. What is an active galaxy? List the different kinds of active galaxies. How do they differ from one another?
4. What is a supermassive black hole? What evidence is there to support the idea that supermassive black holes are located at the centers of many galaxies?